1. Field of the Invention
The present invention relates to systems and methods for detecting objects, and more particularly, to systems and methods for detecting objects to control a vehicle.
2. State of the Art
Systems have been developed which use a low power diode laser at near infrared wavelengths for vehicle (e.g., automobile) control. An exemplary system is described in "Design Method for an Automotive Laser Radar System and Future Prospects for Laser Radar" by Sekine et al. in Proceedings of the Intelligent Vehicles '92 Symposium, Jun. 29-Jul. 1, 1992, Detroit, Mich., pp. 120-125.
Several problems commonly arise when these systems are used for detecting objects such as retro-reflector markers located along a path over which a vehicle is to be guided. For example, to ensure the detection of the objects, a beam must be generated with a relatively large field of view (i.e., wide beam path). The field of view of a beam is typically widened by changing the focus of a transmitter lens or by splitting the beam into a plurality of lower intensity beams. For example, U.S. Pat. No. 4,902,126 (Koechner) discloses an obstacle avoidance system where a field of view is increased by splitting a laser beam into a plurality of beams of lower power.
A problem with the foregoing approach is that achieving wider fields of view by splitting the original beam or by defocusing the beam typically results in an undesirable decrease in the radiation received by an object in the field. Thus, although the field of view may be expanded by increasing the "blur spot" (e.g., beam that is 40% wider and taller at the expected location of an object), the intensity of the field decreases and the reflections become difficult to detect. For example, detection using slower CCD technology with prolonged integration time would be susceptible to noise (e.g., sunlight).
An alternate solution reduces the need for a wide field of view by aligning the system to operate in a set configuration whereby the field of view is relatively fixed with respect to an expected location of a retro-reflector. However, a system configuration is rarely fixed. For example, a change in vehicle (e.g., automobile) load by adding or subtracting passengers to the back seat, or otherwise changing the weight in the vehicle, causes the angle of the transmitted beam to change relative to the expected location of an object such that a marker formerly within the beam's field of view may no longer be in the field of view.
When the intensity of the beam is increased to accommodate an increased field of view (i.e., so that distant objects in the field can still be detected), problem arise with detecting objects close to the beam source (i.e., near field signals). The increased intensity of a signal at close distances from the vehicle may be so substantial that locations along a vehicle path where no reflector is present can cause a false return. A false return can adversely affect the vehicle guidance control, causing the vehicle to stray from the desired vehicle path.
One solution to this problem, as described in the Sekine et al. article, reduces the signal by turning down the gain for the signal during the early time (i.e., fast time) of the return. This method however, is only effective for a system which has a fast time response. In a system which measures azimuth angles to guide a vehicle there may be no way to inhibit the early return signals.